Catheter with electrodes for impedance and/or conduction velocity measurement

ABSTRACT

An apparatus comprises a catheter comprising a first electrode. The apparatus also comprises a second electrode electrically attached to a person and coupled to the first electrode via the person&#39;s tissue. Logic is coupled to the electrodes and generates an electrical signal that is provided through the electrodes and computes an impedance or conduction velocity associated with the electrodes based on the electrical signal. The logic stores a threshold against which said computed impedance or conduction velocity is compared by said logic

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional application Ser. No.61/019,131 entitled “Method, System, and Device for Detection ofPericardial Blood or Fluid” and incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable

BACKGROUND

1. Field of the Disclosure

This invention relates generally to the field of medical devices. Morespecifically, the invention relates to a method and device usingimpedance for the detection of fluid (e.g., blood) bleeding such aspericardial effusion, retroperitoneal effusion, etc.

2. Background Information

Radiofrequency ablation (RF ablation) or other invasive cardiacprocedures which involve operation within the cardiac chambers, coronaryarteries or the heart's venous anatomy have saved many lives. Theseprocedures often involve percutaneous access into the cardiac chambersor epicardial arterial or venous vessels. Catheter, pacing lead, sheath,or other types of device manipulations frequently are performed as keyparts of these procedures. Example of this include balloon angioplastyor stent placement. Often, catheter access to the femoral artery isneeded.

A rare but potentially dangerous complication of these and similarprocedures is inadvertent perforation of a cardiac chamber or anepicardial vessel. Retroperitoneal bleeding is also possible at the siteof the insertion of the catheter into the femoral artery. Perforationsof a cardiac chamber or an epicardial vessel may lead to accumulation ofblood (or other fluids) in the pericardial space or sac. This conditionis referred to pericardial effusion. Cardiac tamponade is thepatho-physiologic state wherein accumulation of blood or other fluid inthe pericardial space or sac leads to impaired filling of the heart anda secondary decrease in cardiac output and consequential hemodynamicderangement. It is not unusual in clinical procedures for the onset ofperforation to be heralded by the onset of hemodynamic derangements suchas drop in blood pressure. In such cases it is frequently only at thattime that the presence of a perforation is recognized. Much time mayhave elapsed between the creation of a perforation and the subsequentaccumulation of enough blood or fluid to create ahemodynamically-significant pericardial effusion or tamponade. Ofcritical clinical significance is that early detection of suchperforation may allow the operator to implement interventions (forexample discontinuation of peri-operative anticoagulation) that wouldmitigate the untoward consequences of pericardial effusion.Retroperitoneal bleeding may lead to hemotoma formation, pain, bloodloss, shock, or death. Its detection is frequently only noted afterhypotension or other symptoms are noted. Because of the location ofblood collection (in the retroperitoneal space) there are usually noother signs commonly associated with bleeding such as hematoma orecchymosis formation. As in the case of a pericardial effusion promptrecognition offers the opportunity for potentially lifesavingintervention. Another frequent complication of such procedures involvesdevelopment of blood clots (“thrombosis”) within the body of the sheath.These clots may travel (“embolize”) via the circulation and lead tonecrosis or ischemia of tissue subserved by these blood vessels.

It follows that a method and device which could more rapidly detect thepresence of pericardial or retroperitoneal bleeding, prior to the onsetof tamponade, is highly desirable. Rapid detection of such blood orfluid accumulation can lead to more timely management—such as abortingthe procedure or reversal of the patient's anticoagulation responseduring such cardiac procedures.

BRIEF SUMMARY

In accordance with at least one embodiment, an apparatus comprises acatheter comprising a first electrode. The apparatus also comprises asecond electrode electrically attached to a person and coupled to thefirst electrode via the person's tissue. Logic is coupled to theelectrodes and generates an electrical signal that is provided throughthe electrodes and computes an impedance or conduction velocityassociated with the electrodes based on the electrical signal. The logicstores a threshold against which said computed impedance or conductionvelocity is compared by said logic.

A method embodiment comprises coupling a first electrode to a person,coupling a second electrode to the person, and injecting, by a currentsource, an electrical current that runs through the electrodes. Further,the method comprises computing, by logic, an impedance or conductionvelocity based on the injected current, and determining, by the logic,whether the impedance or conduction velocity is at a level indicative ofbleeding or a clot. The first and second electrodes, respectively, areplaced at sites selected from a group consisting of coronary sinus andesophagus, heart and esophagus, skin and esophagus, coronary sinus andskin, heart and skin, skin and skin, femoral artery and skin, guide wireand skin.

Yet another embodiment comprises an apparatus that includes a cathetercomprising a first electrode, a second electrode electrically attachedto a person and coupled to the first electrode via the person's tissue,and a signal generator coupled to the electrodes. The signal generatorgenerates an electrical signal that is provided through the electrodesand computes an impedance associated with the electrodes based on theelectrical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an introducer sheathe with an electrode usable to determineimpedance for the detection of bleeding in accordance with variousembodiments.

FIG. 2 shows a view of the sheathe with a partial ring electrode on anexterior surface in accordance with various embodiments.

FIG. 3 shows a view of the sheathe with a complete ring electrode on theexterior surface in accordance with various embodiments.

FIG. 4 shows a view of the sheathe with an electrode embedded in thematerial of the sheathe, in accordance with various embodiments

FIG. 5 shows a view of the sheathe with an electrode on an interiorsurface of the sheathe in accordance with various embodiments.

FIG. 6 depicts the sheathe inserted into a blood vessel of person andconnected to an impedance measuring apparatus in accordance with variousembodiments.

FIG. 7 shows a method in accordance with various embodiments.

FIG. 8 shows an impedance measuring apparatus in accordance with variousembodiments.

FIG. 9 illustrates various impedance thresholds stored in the impedancemeasuring apparatus.

FIG. 10 depicts an illustrative method of calibrating the impedancemeasuring apparatus.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. This document does not intendto distinguish between components that differ in name but not function.

In the following discussion and in the claims, the term “fluid” isdefined to include blood and other types of body fluids that may bleedfrom a vessel or organ and that the disclosed technique detects.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with preferred embodiments of the invention, a system andmethod are disclosed herein that involves real-time assessment ofresistance or impedance to an electrical signal (current or voltage).Accumulation of sufficient fluid or blood in such areas as thepericardial space leads to changes in both the direct current (DC)resistance and/or the complex impedance to alternating current (AC)current flow. A change in either the resistance or the complex impedancesignals fluid accumulation in the space through which the electricalcurrent travels. Embodiments of the invention also use conduction timebetween two vectors as another variable which may be analyzed. Variousembodiments are described herein for measuring impedance to detect fluidbleeding.

In accordance with one such embodiment, FIG. 1 illustrates an introducer10 usable to insert a catheter into a blood vessel (vein or artery). Theintroducer comprises a hollow sheathe 12 having a distal end 19 that isinsertable into a blood vessel of a person. The blood vessel may be anartery or a vein. In at least one application, the blood vessel is thefemoral artery, but other blood vessels may be used as well. In theillustrative embodiment of FIG. 1, an electrode 20 is provided on thesheathe 12 near distal end 19. The electrode can be provided, however,at other locations along the sheathe 12. As will be explained below, theelectrode is usable to measure impedance of the person so as to detectbleeding (e.g., retroperitoneal bleeding). Impedance between pairs ofelectrodes within the sheath can also be measured to assess the presenceof such phenomenona as clots within the sheath.

The sheathe 12 may be coupled to a hub which may or may not incorporatea hemostasis valve 21 from which a side arm 4 may extend that allows tosheathe to be used to administer fluids and or drugs. A valve 16 isprovided on the opposing end of the side arm 14. The introducer 10 alsoincludes a dilator 28 that is insertable into the hollow sheathe 12. Thedilator and sheathe function to permit a catheter to be inserted intothe blood vessel.

Referring still to FIG. 1, an electrical conductor 17 (e.g., a wire)extends along at least part of the sheathe 12 from the electrode 20 andcan be connected to an external device (i.e., a device external to theperson/patient that receives the sheathe 12). The conductor 17 is usableto conduct signals between the electrode 20 on the sheathe and theexternal device for impedance measurements.

In FIG. 1, a single electrode 20 is shown on the sheathe 12, but inother embodiments, more than one electrode is provided on the sheathe.Impedances between any individual electrodes may be measured.

FIGS. 2-5 illustrate various embodiments of the electrode 20. Eachfigure shows a view of the sheathe facing distal end 19. Referring firstto FIG. 2, sheathe 20 comprises material 24 formed as a tubular memberand comprising an inner surface 23 and an outer surface 25. In FIG. 1,the electrode 20 comprises a partial ring electrode disposed about aportion of the perimeter of the outer surface 25. In some embodiments,the electrode 20 is adhered (e.g. via glue) to the outer surface 25. Inother embodiments, the electrode 20 covers more than 50% of theperimeter of the outer surface 25 and is retained (e.g., clamped) inplace like a bracelet.

FIG. 3 illustrates an embodiment of the electrode 20 in which theelectrode is a complete ring electrode (i.e., completely surrounds theouter surface 25 of the sheathe 12).

In FIGS. 2 and 3, the electrode 20 is provided on the outer surface 25of the sheathe. In FIG. 4, the electrode 20 is embedded within thematerial 24 of the sheathe in which case the sheathe materials (or atleast the segments of the sheath material between the electrodes) mustbe conductive of electrical signals in the range employed. Furthermore,for the purposes of detection of clot, impedance or conduction betweenthese electrodes may be measured. In FIG. 5, the electrode 20 isprovided on the inner surface 23 of the sheathe and thus within theinner hollow portion of the sheathe.

In some embodiments, the electrode 20 is located on the distal end 19 ofthe sheathe so that the electrode will be inside the blood vessel oncethe sheathe is inserted into the vessel. In other embodiments, theelectrode may be provided on the sheathe at the proximal end outside theblood vessel (and perhaps even outside the person's body). In suchembodiments, the electrode 20 preferably is provided on the innersurface of the sheathe (similar to that shown in FIG. 5). Normally, thesheathe is filled with body fluid (e.g., blood).

FIG. 6 illustrates a person lying supine with the sheathe 12 insertedinto a blood vessel 29. The conductor 17 from the electrode 20 isconnected to an impedance measuring apparatus 35. The impedancemeasuring apparatus 35 comprises a current source 36 and logic 38. Thecurrent source 36 may be part of the logic 38 if desired. A secondelectrode 30 is also connected to the impedance measuring apparatus 35.The current source 36 injects an electrical current through one of thetwo electrodes 20 or 30. The current then passes through the person'stissues, into the other electrode and back to the current source. Theinjected current may comprise a series of pulses or a sustained current.The amplitude of the current may be 1 milliamps. If a pulse train isused, the pulse width may be 0.2 milliseconds or less and have afrequency of between 5,000 and 500,000 Hz or higher.

The current source 36 or logic 38 measures the voltage across theelectrodes 20, 30 resulting from the current, and computes the ratio ofthe voltage to current to compute impedance. The impedance is altered inthe presence of bleeding and thus can be correlated to bleeding such asretroperitoneal bleeding. The device may also calculate the conductiontime between the electrodes. Bleeding will also alter the conductiontime between tissues.

The second electrode 30 may be located at any of a variety of locations.The illustrative embodiment of FIG. 6 shows the second electrode 30attached to the skin on the person's back as a patch electrode. In otherembodiments for detecting retroperitoneal bleeding, the second electrode30 can be attached to a urinary catheter, a rectal temperature probe, anelectrosurgical grounding pad, or a patch on lateral aspect of the backas desired.

FIG. 7 shows an illustrative method. At 102, an electrode is coupled tothe person. In the embodiments of FIGS. 1-6, such an electrode islocated on an introducer sheathe 12 and coupled to a person as thesheathe is inserted into a blood vessel. At 104, a second electrode isalso coupled to the person (e.g., back electrode 30 as shown in FIG. 6).At 106, upon a user activating a control on the impedance measuringdevice to begin the impedance measuring activity, the impedancemeasuring apparatus 35 injects current and, at 108, computes theimpedance (e.g., measures the voltage and computes the ratio of voltageto current).

At 110, the impedance measuring apparatus 35 determines if the impedanceis indicative of bleeding. In some embodiments, the logic 38 of theimpedance (or conduction time) measuring apparatus 35 compares thecomputed impedance to a predetermined threshold, derived theresholdbased on baseline measurements at the onset of the procedure, otherwisedefined acceptable range. The logic 38 determines that bleeding hasoccurred if the computed impedance or conduction time is outside of theacceptable range for the threshold as previously defined. If bleedinghas been detected, the logic 38 may alert a user via an audible and/orvisual indicator.

In some embodiments, the impedance measuring apparatus 35 injects aknown current and measures the resulting voltage to determine impedance.In other embodiments, the impedance measuring apparatus 35 applies aknown voltage to the electrodes and measures the resulting current todetermine impedance.

It may be desirable to leave the sheathe 12 in place in the person'sblood vessel following the completion of the medical procedure (e.g., RFablation) for which the sheathe was used in the first place. It ispossible that bleeding (e.g., retroperitoneal bleeding) will begin afterthe completion of the medical procedure. With the sheathe 12 still inplace, impedance measurements can be made via the impedance measuringapparatus 35 to detect post-medical procedure completion onset ofbleeding. A user of the impedance measuring apparatus can activate acontrol (e.g., press a button) on the impedance measuring apparatus toactivate an impedance/bleed monitoring.

Besides retroperitoneal bleeding, other types of internal bleeding mayoccur as well. For example, during a catheterization procedure of apatient's heart or surrounding blood vessel(s), bleeding can occur intothe pericardial space. In accordance with various embodiments, acatheter includes one or more electrodes, at least one of which is usedto make impedance measurements as described above to detect bleedingsuch as pericardial effusion. In another embodiment of this inventionthe tip of the catheter or electrode may be located on any guide wireused during coronary intervention (a wire over which a coronary stent orangioplasty apparatus may be advanced is always utilized during suchprocedures). In this embodiment, the guide wire is or contains anelectrode. In such a situation the impedance between the tip of the wireand any second electrode as described elsewhere (such as a skin patchelectrode) can be utilized. In another embodiment a distal and proximalelectrode (relative to the location of coronary blockage which is to beangioplastied or stented) within the same wire may be used to assessprogression of clot formation or perforation and effusion.

FIG. 8 illustrates an embodiment of an impedance measuring apparatus 150usable to measure impedance and detect bleeding. Any of the attributesdescribed below for impedance measuring apparatus 150 can apply toimpedance measuring apparatus 35 of FIG. 6 as well. The impedancemeasuring apparatus 150 comprises a processor 152, a signal generator154, an output device 156 and storage 158. The storage 158 comprisesvolatile memory (e.g., random access memory), non-volatile storage(e.g., read only memory, hard disk drive, Flash storage, etc.), orcombinations thereof. The storage 158 comprises an application 159usable to perform impedance measurements and detect bleeding asdescribed herein and calibration software 162. Both applications 159 and162 are executed by processor 152. Storage 158 is also used to store oneor more impedance thresholds 160. The impedance measuring apparatus 150comprises logic which includes any combination or all of the processor152, signal generator 154, and storage 158 (and associated applicationsand thresholds stored thereon).

Electrodes 172 are provided on a catheter 170 and electrically coupledto the signal generator 154. On or more additional electrodes 174 mayalso be provided and coupled to signal generator 154. Under control ofthe processor 152 (via execution of application 159), the signalgenerator 154 selects one pair of electrodes 172, 174, applies a knowncurrent to one of the electrodes in the selected pair, receives thecurrent via the electrode, determines the resulting voltage across theselected pair of electrodes, computes the impedance (ratio of voltage tocurrent), or conduction time and compares the computed impedance orconduction time to a corresponding threshold to determine if bleedinghas occurred. A pair of electrodes can be selected coupling two of theelectrodes 172, 174 to the signal generator (via a switching device).The signal generator can select two electrodes from among electrodes 172on the catheter, two electrodes from among electrodes 174, or oneelectrode each from electrode sets 172 and 174.

If two electrodes 172 are selected on the catheter 170, the impedancemeasuring apparatus 150 can detect a blood clot within the catheter bymeasuring the impedance between the two catheter electrodes. The same istrue with respect to the embodiment of FIG. 1. The sheathe 12, in someembodiments, comprises more than one electrode 20. The impedancemeasuring apparatus 35 measures the impedance between the electrodes onthe sheathe to detect blood clots that may form within the sheathe.

The catheter 170 can be inserted into any of a variety of veins orarteries. In one embodiment, the catheter 170 is inserted into thefemoral artery (for detection, for example, of retroperitonealeffusion), the heart or coronary vasculature such as the coronary sinus(for detection of pericardial effusion), or other blood vessels oranatomic structures. The coronary sinus is an epicardial vein throughwhich venous drainage of coronary circulation occurs. It is on theinferior surface of the left atrium. More distally this structure turnsinto the great cardiac vein or any of its other tributaries.

The electrodes 174 may be located at any of variety of sites. Anelectrode 174, for example, may be located on the person's esophagus, onthe person's skin, or on the person's heart. Moreover, impedance can bemeasured for detecting bleeding between, for example, the coronary sinusand skin, coronary sinus and esophagus, skin and skin (e.g., patient'sfront and back), heart and coronary sinus, heart and esophagus, twosites on the same catheter, two sites on the same sheathe, two sites onthe same vein and femoral artery to skin.

As explained herein, more than two electrodes can be used for measuringimpedance. Impedance can be measured between any pair of electrodes andsuch an impedance measurement represents a vector. For example, in athree-electrode system (first, second, and third electrodes), there arethree possible impedance vectors including the impedance between thefirst and second electrodes, the impedance between the first and thirdelectrodes, and the impedance between the second and third electrodes.The number of vectors increases disproportionately with increasingnumbers of electrodes. The physical location of the various electrodesmay be useful to detect bleeding in different locations. For example,bleeding may occur between the first and second electrodes, but thefluid (e.g., blood) may not be present between the second and thirdelectrodes. Thus, in this example, the impedance vector associated withthe first and second electrodes may be indicative of the bleed, but notso the impedance vector associated with the second and third electrodesor possibly the first and third electrodes. Moreover, more than twoelectrodes provides an enhanced ability to detect bleeding in differentlocations than might be possible in a two-electrode only system.

In some embodiments, the computed impedance may be resistance while inother embodiments, the computed impedance is complex having bothamplitude and phase components. In other embodiments the computedvariable is conduction velocity. Further, the impedance measuringapparatus 150 (or impedance measuring apparatus 35 in FIG. 6) determinesand stores an impedance threshold for each impedance vector. Two or moreof the various impedance thresholds may be the same or the impedancethresholds may all be different. Each impedance threshold may be anamplitude only value (resistance) or, in the case of complex impedance,comprise an amplitude value and a phase value.

FIG. 9 illustrates the thresholds 160 as a table comprising one or morevectors A, B, C, etc. Each vector represents a pair of electrodes. Foreach vector, there is an amplitude threshold value 166 and/or a phasethreshold value 168. In some embodiments, the impedance measuringapparatus detects the presence of bleeding if either of the amplitude orphase of the computed impedance for a given vector exceeds itscorresponding amplitude or phase value. In other embodiments, theimpedance measuring apparatus detects a bleed only if both the computedamplitude and phase exceed their corresponding threshold counterparts.The threshold counterparts may have been derived in a variety of waysone of which may be baseline measurements at the beginning of theprocedure for each individual patient as explained below.

FIG. 10 illustrates a method 200 for calibrating the impedance measuringapparatus (35 or 150) for the various thresholds. In some embodiments,the impedance measuring apparatus comprises a calibration mode that canbe initiated by a user of the impedance measuring apparatus (e.g., bypressing a button). The processor of the impedance measuring apparatusexecutes the calibration software 162 (impedance measuring apparatus 35may also have similar software to be executed by a processor). FIG. 10is a method performed by the process upon executing the calibrationsoftware 162. The calibration mode is performed preferably before themedical procedure [e.g., coronary angiography (after the wire has beenplaced through the blockage but before angioplasty) or electrophysiologystudy (after catheters have been placed in the coronary sinus but beforedelivery of radiofrequency ablation)] begins.

The calibration mode begins at 202. A pair of electrodes is selected at204 and at 206 and 208, an impedance measurement is taken and thecomputed impedance is recorded (e.g., stored in storage 158) (asamplitude and/or phase values). Preferably, the impedance measurementfor a selected pair of electrodes is taken over the course of severalbreaths by the patient. The impedance computed for the selectedimpedance vector will vary during a respiratory cycle. By taking theimpedance measurement over the course of several breaths (e.g., 10seconds), the impedance measuring apparatus can account for the normalvariations in impedance. The threshold (amplitude or phase) may becomputed as an average during the recording period or may be set as thepeak value detected (or a value slightly higher (e.g., 5% higher) thanthe peak). At 210, the impedance measuring apparatus determines whetherthere is an additional impedance vector for which a threshold is to bedetermined. If there is, control loops back to step 204 at which such anelectrode pair is selected. If not more electrode pairs are to beselected, than the calibration mode stops at 212. This calibrationprocess may take several minutes. The same calibration variables may bemeasured for conduction velocities.

Once the calibration process is completed, the medical procedure (whichmight result in bleeding or clot formation) can begin. Any bleeding willbe detected a change in impedance above deviating from an impedancethreshold (e.g., an increase above the threshold or decrease below thethreshold).

The impedance measuring techniques described herein to detect bleedingare also usable to detect a hemothorax. In this application, electrodelocations would include the anterior chest and posterior chest walls,the esophagus at the level near the heart, the trachea, as well asnumerous intravascular and intra-cardiac and intra-coronary locations.The electrodes may be on catheters or wires.

With regards to conduction velocity, the logic (e.g., that contained inthe measuring devices described herein) assesses the conduction timebetween the onset of the electrical impulse in the first (transmitting)electrode and second (receiving) electrode. These electrodes areidentical to the electrodes described in this invention. The electricaloutput is in the same range with regards to frequency and amplitude. Themeasured variable, however is the difference (delta) in time (usuallymilliseconds) between onset of stimulus (electrical output) in thetransmitting electrode and sensing of that impulse (electrical sensing)in the receiving electrode. Conduction velocity is heterogeneous withvariations in tissue characteristic. As fluid develops, the conductionvelocity between the transmitting and receiving electrode will alsochange. This will be noted as a deviation from a baseline values(similar to the impedance values/thresholds described herein). An alertwill then be issued. The various embodiments of apparatus and methodsdescribed above can also be used to measure conduction velocity and useconduction velocity to determine thickening of the heart and thepresence of fluid bleeding.

While the embodiments of the invention have been shown and described,modifications thereof can be made by one skilled in the art withoutdeparting from the spirit and teachings of the invention. Theembodiments described and the examples provided herein are exemplaryonly, and are not intended to be limiting. Many variations andmodifications of the invention disclosed herein are possible and arewithin the scope of the invention. Accordingly, the scope of protectionis not limited by the description set out above, but is only limited bythe claims which follow, that scope including all equivalents of thesubject matter of the claims.

The discussion of a reference in the Background Information is not anadmission that it is prior art to the present invention, especially anyreference that may have a publication date after the priority date ofthis application. The disclosures of all patents, patent applications,and publications cited herein are hereby incorporated herein byreference in their entirety, to the extent that they provide exemplary,procedural, or other details supplementary to those set forth herein.

1. An apparatus, comprising: a catheter comprising a first electrode; asecond electrode electrically attached to a person and coupled to saidfirst electrode via the person's tissue; and logic coupled to saidelectrodes, said logic generates an electrical signal that is providedthrough said electrodes and computes an impedance or conduction velocityassociated with said electrodes based on said electrical signal; whereinsaid logic stores a threshold against which said computed impedance orconduction velocity is compared by said logic.
 2. The apparatus of claim1 comprising a third electrode and said logic computes impedancesbetween said first and third electrodes and said second and thirdelectrodes, said logic also stores separate thresholds against whichsaid impedances are compared.
 3. The apparatus of claim 2 wherein thelogic implements a calibration mode in which said thresholds aredetermined, said calibration mode initiated before a medical procedureis begun.
 4. The apparatus of claim 3 wherein, while in the calibrationmode, the logic determines an impedance between each pair of the first,second, and third electrodes.
 5. The apparatus of claim 1 wherein thelogic implements a calibration mode in which said threshold isdetermined, said calibration mode initiated before a medical procedureis begun.
 6. The apparatus of claim 1 wherein the threshold comprises animpedance amplitude value.
 7. The apparatus of claim 1 wherein thethreshold comprises an impedance phase value.
 8. The apparatus of claim1 wherein the threshold comprises an impedance phase value and animpedance amplitude value.
 9. The apparatus of claim 1 wherein saidlogic computes said impedance with the catheter inserted into thecoronary sinus of the person.
 10. The apparatus of claim 1 wherein thesecond electrode is at a site selected from a group consisting of theperson's skin, the person's esophagus, and the person's heart.
 11. Amethod, comprising: coupling a first electrode to a person; coupling asecond electrode to the person; injecting, by a current source, anelectrical current that runs through said electrodes; computing, bylogic, an impedance or conduction velocity based on said injectedcurrent; and determining, by said logic, whether said impedance orconduction velocity is at a level indicative of bleeding or a clot;wherein the first and second electrodes, respectively, are placed atsites selected from a group consisting of coronary sinus and esophagus,heart and esophagus, skin and esophagus, coronary sinus and skin, heartand skin, skin and skin, femoral artery and skin, guide wire and skin.12. The method of claim 11 wherein determining, by said logic, whethersaid impedance is at a level indicative of bleeding comprisesdetermining whether the impedance is at a level indicative ofpericardial effusion or a hemothorax.
 13. The method of claim 11 furthercomprising performing a calibration in which a threshold is computed forimpedance between said first and second electrodes.
 14. The method ofclaim 13 wherein the computed threshold comprises a phase threshold. 15.The method of claim 13 wherein the computed threshold comprises a phasethreshold and an amplitude threshold.
 16. The method of claim 11 furthercomprising coupling a third electrode to the person and computingimpedance associated with said first and third electrodes and saidsecond and third electrodes.
 17. The method of claim 16 furthercomprising determining a threshold for the impedance computed for thefirst and second electrodes, the first and third electrodes, and thesecond and third electrodes.
 18. An apparatus, comprising: a cathetercomprising a first electrode; a second electrode electrically attachedto a person and coupled to said first electrode via the person's tissue;and a signal generator coupled to said electrodes, said signal generatorgenerates an electrical signal that is provided through said electrodesand computes an impedance associated with said electrodes based on saidelectrical signal.
 19. The apparatus of claim 18 wherein the signalgenerator annunciates an alert if the computed impedance devices from atarget impedance.
 20. The apparatus of claim 18 wherein the cathetercomprises a plurality of electrodes which are selectable by the signalgenerator to compute impedance.